The development of the process for the polymerization in gas phase reactors led to the production of a wide range of polymers. The use of a polymerization process in a gaseous fluidized bed significantly lowers energy requirement and capital investment as compared to other processes. A limiting factor in the polymer production rate in a gas phase reactor is the maximum rate at which the exothermic heat of the reaction may be removed from the reactor. Usually, heat removal in gas phase processes is effected through the compression, cooling and reintroduction of a fluid medium in the recycle, usually a gas, which belongs to the reaction system, so that such fluid not only works as the fluidizing medium of the reactor, but also serves as a dispersing heat medium for removing the heat generated within the reactor. Under the point of view of heat exchange, it is advisable that the flow rate of gas be the highest possible. However, in order to prevent the excessive entrainment of solid particles in the gaseous stream withdrawn from the reactor, the linear speed of the gaseous stream related to the cross section of the reactor should not exceed certain limits.
Therefore, one way to maximize the ability of heat removal is, throughout the operation, to reduce to the lowest possible value the temperature of the gaseous feed stream into the reactor. Beforehand it was estimated that the temperature of the recycle gas could not be lowered to values close to the dew point of the stream. The dew point of the recycle stream is that temperature at which the condensation of liquid in the stream starts. It was believed that the introduction of a partially liquefied stream into a gas phase reactor would inevitably result in a series of operating drawbacks and, at extreme situations, where excessive liquid portions would be present in the cooled recycle stream, would lead to the collapse and the solidification of the bed. A stable operation condition of such kind of reactor is characterized by a bed of moving suspended particles which are thoroughly mixed and in a stable state, even when under reactive conditions, without formation of significant amounts of agglomerated particles that may harm the operation of the reactor or of the process steps ahead.
Contrary to such statements, it was found that, under certain conditions, a recycle stream might be cooled to a temperature below its dew point, hence the condensation of a fraction of said recycle stream. Under such conditions, such stream may be reintroduced in the reactor without the occurrence of the operating drawbacks mentioned hereinbefore.
In gas phase polymerization processes, said form of deliberate introduction of a partially condensed recycle stream into the reactor, together with the judicious choice of conditions that provide a continuously stable operating mode is known as operation in the “condensed mode”.
The state-of-the-art technique as cited by the patents on the condensed mode operation uses nitrogen in appreciable amounts as the non-volatile constituent of the recycle stream, ethane and methane being also present in smaller amounts. Alkanes such as iso-pentane and n-hexane are typically employed as constituents of relatively low volatility in concentrations, which may induce the required partial condensation.
U.S. Pat. No. 4,543,399 and U.S. Pat. No. 4,588,790 report the condensed mode operation in the production of polyolefins. Said documents indicate further that there are restrictions related to the process operation ability, mainly as regard the limits of the liquid content in the recycle stream throughout the condensed mode operation. The typical suggested range for the condensed mode operation is situated between 2% and 12% by weight of condensed liquid present in the recycle stream.
More recently, U.S. Pat. No. 5,352,749 and U.S. Pat. No. 5,436,304 teach that, under certain more specific and restrictive conditions, it is possible to operate in the condensed mode in the presence of an amount of condensed liquid higher than 17.4% by weight of the recycle stream. Such potential operation mode is known as “super-condensed mode”. Its use is motivated by the desire to increase still more the ability of heat removal and therefore to attain production rates ahead of the accepted levels of operation according to the condensed mode as described hereinbefore.
Thus, it can be seen that the as-described gas phase polymerization processes aiming at obtaining high production rates use the condensed mode and super-condensed mode, this entailing a few negative aspects.
It is well known that, while the operation in the condensed and super-condensed modes is beneficial in terms of higher production rates of the reactor, it presents very often a few drawbacks caused by the presence of condensed liquid in the recycle stream. Thus, in certain operating variations of the condensed mode, a non-reactive and relatively non-volatile hydrocarbon such as iso-propane or n-hexane is injected in the reaction circuit in order to promote the condensation.
The presence of such constituents, especially in relatively high amounts, may promote the swelling and adhesion of the polymer particles and tends to be slightly conflicting with the maintenance of a stable process operation. Thus, unfortunately, the state-of-the-art technique employed to promote the levels of condensation in the recycle stream that are required to enter the super-condensed regimen requires such amounts of non-volatile constituents that may harm the stable operation of the reactor. In practical terms, in order to reach higher production rates, the attainment of a higher heat removal ability is not sufficient, since it is also essential to keep a stable operation. In case the stable operation cannot be kept as such, the heat removal ability is no longer the limiting factor for the production rate, while the advantage of the operation in the condensed and super-condensed modes is lost. The difficulties in keeping the process stability in the super-condensed mode as well as the ways of keeping pace with those drawbacks are thoroughly discussed in the said U.S. Pat. No. 5,352,749 and U.S. Pat. No. 5,436,304 mentioned before.
In further variations of the gas phase polymerization process, where comonomers of longer chains and consequently lower volatility such as 1-pentene, 1-hexene or 1-octene are employed in relatively high amounts for copolymerizating with the primary monomer, due to problems related to the suitable heat removal and to the stability of the reactor bed; the combination of the process conditions with the features of such monomers may lead to the partial condensation of said comonomers and to the consequent impossibility to operate the process under high production rates.
On the other hand, a few patents focus on the non condensated operation by modifying the composition of the recycle stream without however any mention nor claim of an increase in the production rate at levels that would approach those of the condensed mode operation.
This way, U.S. Pat. No. 4,469,855 teaches a process for the copolymerization of ethylene in gas phase reactors, where the diluents used are inert gases such as helium, argon, nitrogen or especially ethane. This process is favorably applied for the copolymerization of comonomers having 6 or 8 carbon atoms.
Potential sources of inert gas diluents for concentrating in the reactor may depend specifically on the type of gas. For example, U.S. Pat. No. 5,681,908 teaches of a process, which may provide a source of ethane and propane to the reactor, via the recycling of monomers and saturated hydrocarbons from reactor purge streams. The process comprises a collection of equipment external to the fluidized bed polymerization process, which via methods of absorption and distillation, is able to recover inert gases such as ethane and propane for recycle to the polymerization reaction system. Such an external system may represent a suitable source of inert gases to enable the practice of the present invention.
U.S. Pat. No. 4,525,547 teaches a process for producing copolymers of ethylene and olefins having of from 4 to 10 carbon atoms in gas phase reactors, the process comprising re-circulating and cooling without liquefying the gaseous stream exiting the reactor. In said process, the composition of the gas phase contains as a replacement to nitrogen, a saturated gaseous hydrocarbon, in an amount higher than 1 mole per mole of ethylene, so as to promote a uniform copolymerization while avoiding the condensation of monomers during the cooling of the recycle stream. The saturated gaseous hydrocarbon has between 3 and 6 carbon atoms.
U.S. Pat. No. 5,733,987 teaches a three-stage process for obtaining ethylene polymers and copolymers. Catalyst contact and pre-polymerization steps precede the gas phase step. In the gas phase reactor, the mole composition of the re-circulation stream contains of between 20 and 90% alkanes based on the total gases. The alkanes used have between 3 and 6 carbon atoms. Such modification allows a better reaction control and prevents the formation of aggregated particles.
The feature of such processes, which apply the non condensed mode in the presence of an alkane-containing atmosphere having more than 3 carbon atoms as a replacement to nitrogen, is that they require a high ratio of said alkanes to the gas phase ethylene so as to prevent the condensation of the comonomers. Another drawback is that said processes are limited to the production of polyethylene and its copolymers.
Thus, the patent literature does not describe nor suggest polymerization processes in gas phase reactors leading to the maximization of the limit of the heat removal ability by the recycle stream, and the ensuing maximization of the reactor productivity, while essentially operating in the non condensed mode. Said advantages may be obtained by using a mixture of inert diluents containing at least one light component designed to regulate the dew point and at least one component of intermediate volatility so as to maximize the heat removal ability in the non condensed mode, as a function of the minimum possible cooling temperature of the recycle stream. A process having such features, suitable for polymerizing ethylene and its copolymers as well as propylene and its copolymers, in gas phase reactors which operate in the non condensed mode and with production rates significantly higher than in the conventional mode of non condensed operation, at the same time equivalent or higher than the rates reached by the operation in the condensed mode, while at the same time keeping the stability of the reactor, is described and claimed in the present application.